Threonine Leaching Deficit Overlooked by Formula Tables in Low-Fishmeal Whiteleg Shrimp Feed

Low-fishmeal formulations for Litopenaeus vannamei increasingly replace fishmeal with solvent-extracted soybean meal, pea protein concentrate, and corn gluten meal; these formulas typically require supplemental crystalline L-threonine to satisfy essential amino acid targets. The formula table generated by least-cost formulation software reports total threonine as a dry-matter concentration after ingredient assembly, but the commercial pellet encounters pond water before ingestion because whiteleg shrimp feed on slowly sinking or tray-placed pellets over a period that can exceed 60 min in intensive pond systems. Immersion in static or low-agitation seawater initiates dissolution and diffusion of free amino acids from the pellet matrix. Total amino acid methods such as AOAC 994.12 and ISO 13903:2005 are performed on dry ground feed samples and do not subject the intact pellet to an aqueous extraction stress. The threonine leaching deficit is therefore defined as the difference between the total threonine printed in the formula table and the threonine retained in the pellet after the immersion interval that corresponds to production feeding behaviour. For a whiteleg shrimp grower feed formulated to 1.6–1.8% total threonine, a loss of 15–25% over 45 min would reduce effective dietary threonine by 0.24–0.45 percentage points. This magnitude is nutritionally relevant because threonine is frequently limiting in low-fishmeal shrimp feeds and may be third-limiting after methionine and lysine, depending on protein source ratio. Formula tables therefore describe the dry-matter inventory of the pellet but not the threonine intake that survives immersion.

What Limits the Utility of Total Threonine Tables in Low-Fishmeal Whiteleg Shrimp Grower Diets?

In a least-cost formulation report, the total threonine value is derived from ingredient composition databases that report amino acid levels in dry raw materials and from the assayed or declared content of supplemental L-threonine. These databases are static and do not contain a kinetic term for aqueous leaching. When a formula table shows total threonine at 1.75% in a diet containing 10% fishmeal and 30% soybean meal, the table does not distinguish threonine that is protein-bound within soybean meal from free threonine added as crystalline L-threonine. Crystalline L-threonine has a reported water solubility of approximately 90 g/L at 25 °C and a molecular weight of 119.12 g/mol; these properties place it among the low-molecular-weight solutes that diffuse rapidly through hydrated pore networks. Ingredient digestibility coefficients for threonine in soybean meal or pea protein concentrate describe enzymatic disappearance in the shrimp gut, not physical retention during immersion. A formula table may therefore meet a true digestible threonine target while the retained threonine after immersion is below the same target. The table-based formulation approach assumes that all threonine remains in the pellet until ingestion, but this assumption fails when pellets are not completely water-stable or when free amino acid is concentrated at the pellet surface. Published data for the exact retention half-life of crystalline threonine in whiteleg shrimp pellets across different water temperatures, salinities, and pellet densities is limited. Production-scale feed mill records indicate that high surface deposition of free amino acids can produce measurable reductions in recovered threonine after 30 min of static immersion, particularly when the amino acid is top-dressed after extrusion without a protective film. The formula table is thus a dry-matter inventory rather than an intake prediction.

Pellet Microstructure, Water Ingress, and Pore-Mediated Threonine Efflux

Water ingress into a twin-screw extruded shrimp feed pellet begins at surface fissures, die-face cracks, and incompletely gelatinized starch domains. Commercial twin-screw extruders used for shrimp feeds commonly operate at L/D ratios between 20:1 and 25:1, with preconditioner discharge temperatures of 90–95 °C and extruder barrel temperatures from 110 °C to 135 °C. Specific mechanical energy input can range from 100 kJ/kg to 300 kJ/kg depending on screw profile and die restriction; higher mechanical energy increases starch gelatinization and pellet density, while lower mechanical energy may leave open pore networks that increase hydraulic connectivity between the pellet surface and interior. Pellet density below 600 g/L can accelerate water uptake and solute loss, while smaller pellet diameters from 1.5 mm to 0.8 mm increase surface-to-volume ratio and reduce diffusion path length for juvenile feeds. After extrusion, drying to 8–10% moisture is required for storage stability, but overdrying below 7% moisture can generate surface fissures that act as preferential water uptake channels. Post-conditioning at 90 °C and 85–90% relative humidity for 20–30 min after extrusion is a common method to improve pellet surface sealing. The diffusion of dissolved threonine through hydrated pores is governed by pellet porosity, tortuosity, and binder swelling. Water-soluble binders such as gelatinized wheat starch, pre-gelatinized tapioca starch, and carboxymethyl cellulose modify the pore network but do not eliminate it. Immersion testing in seawater at 28 °C and 30 ppt salinity for 30 min typically requires dry matter retention above 85% as a production bench mark. Dry matter retention alone, however, can remain high while soluble amino acid efflux occurs, because starch and protein matrices can retain mass even as low-molecular-weight solutes diffuse outward. Selective leaching therefore permits a pelleted feed with acceptable dry matter retention to exhibit a threonine leaching deficit.

To quantify the threonine leaching deficit under production conditions, a feed mill quality-control laboratory pairs total amino acid analysis of the dry pellet with threonine analysis of the immersion leachate and the residual pellet after a defined static immersion. A practical test places 10 intact pellets in 200 mL of synthetic seawater at 28 °C and 30 ppt salinity for 30 min, 60 min, and 120 min. The leachate is filtered through a 0.45 µm membrane and analysed by HPLC with post-column ninhydrin detection or an equivalent amino acid analyser. The residual pellet is freeze-dried, ground, hydrolysed, and analysed according to AOAC 994.12 or ISO 13903:2005. The mass balance calculation accounts for water uptake and dry matter loss. A feed mill that records less than 90% threonine retention after 30 min or less than 80% after 60 min is operating with a formula table that overstates intake. Production-scale twin-screw extrusion lines show batch-to-batch differences in threonine retention of several percentage points when dryer moisture control varies by more than ±0.5%, because final pore structure and surface fissure density are sensitive to moisture removal rate and pellet core temperature. This sensitivity is not represented in formula table calculations, which treat threonine concentration as fixed after mixing.

When Crystalline L-Threonine Is Top-Dressed After Extrusion

When a low-fishmeal formula relies on crystalline L-threonine as a post-extrusion top-dressed powder or as a component of a vacuum-coated oil suspension, the surface area available for dissolution is maximised because the amino acid is not embedded in the starch-protein matrix. Vacuum coating systems operating at reduced pressure can deposit oil-based coating layers of 0.5–1.5% by pellet weight, but if the oil film is incomplete or if the coating emulsion destabilises during storage, exposed threonine crystals dissolve within minutes of pond immersion. Top-dressed powders also segregate during pneumatic conveying and bucket elevator transfer, producing uneven threonine distribution across the finished batch. Sampling from the top, middle, and bottom of a finished product bin can reveal threonine coefficients of variation above 10% when coating distribution is poor. A production-scale observation from twin-screw extrusion lines is that post-extrusion coating at pellet temperatures above 60 °C can cause oil penetration into surface pores, carrying threonine deeper into the pellet, while coating at temperatures below 40 °C can leave a surface film that cracks during cooling. The leaching deficit is therefore highest when crystalline threonine remains on the pellet surface, moderate when it is partially absorbed into the lipid phase, and lowest when it is incorporated into the mixture before extrusion and bound within the gelatinized starch matrix.

Encapsulation, Binder Systems, and the Limits of Reformulation

Replacement of unprotected crystalline L-threonine with encapsulated or matrix-protected forms reduces the leaching deficit but introduces additional formulation and processing constraints. Encapsulation coatings based on hydrogenated vegetable oils, calcium alginate, or ethyl cellulose can slow threonine release, but they may also reduce protein digestibility if the coating resists enzymatic hydrolysis in the shrimp gut. Matrix-protected threonine produced by spray-drying with starch or zein may have particle size and density distributions that segregate from other fine ingredients during mixing. The feed mill verifies that the protected threonine product maintains a particle size range of 150–300 µm to avoid segregation and poor pellet binding. Binder systems based on gelatinized wheat starch at 8–12% inclusion, pre-gelatinized tapioca starch, or carboxymethyl cellulose at 0.5–1.0% improve pellet stability but may increase pellet hardness to the point of reduced palatability for juvenile shrimp. Addition of hydrophilic binders above 2% can increase water uptake and paradoxically accelerate soluble amino acid diffusion if the binder swells and opens new pore channels. The formulator therefore works within a narrow processing window: binder addition is sufficient to achieve water stability without causing excessive hydration-driven pore expansion. Operational boundaries include pre-drying to 8–10% moisture before vacuum coating, maintaining coating chamber vacuum below 200 mbar, and avoiding free crystalline threonine in high-moisture post-conditioning steps above 12% moisture, where dissolution can occur before the pellet reaches the pond. Published data for the exact interaction between binder type and threonine leaching rate in L. vannamei feeds is limited, but production QC records show that the same total threonine formula can produce different retained threonine values depending on coating order and binder hydration.

The rate of threonine leaching is also sensitive to pond water ionic strength, temperature, and agitation. In full-strength seawater ponds at 30–35 ppt, the osmotic gradient between pellet pore water and bulk seawater is smaller than in low-salinity systems, which may slow initial water uptake; however, the dissolved threonine sink remains large because of continuous diffusion into a large water volume. At water temperatures above 30 °C, molecular diffusion coefficients increase and pond water viscosity decreases, increasing efflux. Paddlewheel circulation and mechanical aeration create low-amplitude turbulence that reduces the boundary layer thickness at the pellet surface and accelerates solute exchange. Formula tables do not include pond water temperature, salinity, or circulation as variables, even though these parameters determine the residence time of soluble threonine in the pellet. A feed mill supplying both low-salinity inland ponds and full-strength seawater ponds may therefore need separate retention coefficients for the same formula. The same limitation applies to feeding trays, where pellets are confined but remain submerged, and to demand feeders, where pellets may be intermittently wetted and dried at the waterline.

How Should a Feed Mill Correct Formula Tables for the Threonine Leaching Deficit?

For each production line, formula, and pellet size, a feed mill can correct formula tables by introducing a threonine retention coefficient. The coefficient is the ratio of retained threonine after a defined immersion period to total threonine in the dry pellet. If the dry pellet contains 1.75% total threonine and the residual pellet after 30 min immersion contains 1.40%, the retention coefficient is 0.80. The formula table should report both total and effective threonine after the immersion exposure that corresponds to observed feeding behaviour in production ponds. This correction is valid only for the specific production line, die configuration, and drying profile used in the measurement. If the extrusion line changes from a single-screw to a twin-screw configuration, or if the post-coating system changes from atmospheric top-dressing to vacuum coating, the retention coefficient must be revalidated. The batch release specification for low-fishmeal whiteleg shrimp feed should include a water-stability dry matter retention value of at least 85% after 30 min, a leachate threonine concentration threshold, and a residual pellet threonine concentration threshold. Without such thresholds, the formula table continues to overestimate the threonine available to the shrimp under pond conditions.